There are rumors of Bittrex delisting BTS, and with them gone, there is no centralized exchange I would use to buy BitShares.

Here are the current alternatives to buy BitShares in a more decentralized fashion
OpenLedger
OpenLedger is a company on top of BitShares. They offer a custom Wallet/Exchange cover combined with a Gateway to get non-native coins in the Decentralized Exchange.
Prominent Pair: OPEN.BTC_BTS
Rudex
Similiar to OpenLedger, Rudex is a service on top of BitShares DEX. They offer
Limited coins available but most of them come with ZERO Fees!
Blocktrades
Blocktrades is a shapeshift alternative with roots in BitShares. They offer a web frontend and are available from within the BitShares Platform
Crypto-Bridge
Similiar to OpenLedger and Rudex, a new Gateway/Bridge to BitShares.
They have to work (lower) on market fees to become relevant in my opinion.
Read More: New BitShares Gateway/Bridge
Every coin can be traded against BTS
or any other Pair :)